Why is white considered a pawn up?

According to chess.com's engine white is up a full point between moves 10 to 14 before it becomes equal again. I think I may have some problems playing against d4 queen's gambit type games as black for some reason I'll try find some old losses. 

At move 10: White has more space (central space at that!), and black's bishop on c8 doesn't have much of a future with the b pawn blocked and white controlling the e5 square

At move 13: One of the main positional features is the open files, and white has better access as black's c8 bishop is interrupting heavy piece harmony on the 8th rank

However, your opponent trades one of their best pieces rather than activating the rooks or otherwise looking to improve. One way of figuring out whether to trade is to count the squares each piece contacts. In the position after a3 Bd7, white's dark square bishop contacts 7 squares while black's is in contact with 4......so why trade it!?

As for your play with black, it's a little unambitious out of the opening. 4...Nbd7 is off: It blocks the bishop on c8, defends a knight which isn't attacked, and has no square to go to except b6 which isn't exactly taking over the game. Stockfish gives dxc4, c5, and Bb4 - all of which are hitting the center.

After c6, you're transposing into a semi slav....but one with white's bishop on f4 which is quite nice for white. Usually, white is either stuck behind the e3 pawn or forced to enter complications with Bg5, so it's not necessarily a coup for white, but it's pleasant. 

However, my engine is giving it as basically equal on move 10. Perhaps my potato laptop and outdated stockfish are being fooled by some incredible positional nuance, but black isn't losing. You're just accepting a slightly worse (but solid) position

That's true but in this case it's pretty clear that there's a "human" advantage in the position for white. 

Peter, when white plays Bf4 this is usually a cue for you to be a little more aggressive, and aim for c5 sooner. Many players respond to Bf4 by going for a Tarrasch structure with ...c5 immediately, which allows you to develop the knight on c6.

So by move six you have a very passive, cramped position, even though it's largely self-inflicted. You're sort of automatically making generic QGD moves despite the fact that your opponent is playing the Bf4 line. 

Notice how your knight on b6 is getting in the way - if the N was on d7 still you could have played c5, Nxc5, then b6 and Bb7. One of the challenges of the Bf4 QGD is that it's very hard for you to play e6-e5 to free your bishop, so you probably want to develop her at b7. The knight is now getting in the way there. 

b6 is often a poor square for the knight in these positions unless you're instantly transferring it to d5. 

h6 is an example of you making a generic QGD move. There is no reason for you to play so passively here. White isn't attacking h7 even once, and it's defended twice. Ng5 isn't a threat. So Nbd6 makes sense, offering to trade one of your clumsiest pieces (and also clearing b6 so you can fianchetto). 

So now 13. Rc1 Qe7 (Qxd1 ceded the open file) 14. e4 and white's position is clearly a little better due to his better-coordinated pieces and central space advantage.